Enkephalin‐positive and acetylcholinesterase‐positive patch systems in the superior colliculus have matching distributions but distinct developmental histories

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Abstract
Histochemical stains for acetylcholinesterase activity and enkephalin‐like immunoreactivity both demonstrate a high degree of patterning in the superior colliculus, particularly in the intermediate and deep layers. Both markers occur predominantly in the neuropil of these layers, and both are principally distributed in distinct macroscopic compartments. We report here that patches of heightened acetylcholinesterase activity correspond to patches of high enkephalin‐like immunoreactivity. The two markers thus delineate largely the same domain in the intermediate and deep layers. The most prominent zones of staining for enkephalin‐like peptide and for acetylcholinesterase also coincided in the dorsolateral periaqueductal gray matter. These findings suggest a close interlocking of one or more acetylcholinesterasecontaining systems with one or more pathways related to endogenous opioids in the superior colliculus. As the acetylcholinesterase expression in the patches is known to match in detail choline acetyltransferase expression, our results also suggest the possibility of local cholinergicopiatergic interactions. In some sections, blood vessels associated with enkephalin‐rich and acetylcholinesteraserich patches extended beyond the colliculus into the periaqueductal gray matter, where they again became surrounded by dense fibrous labeling. This pattern suggests that neurohumoral signal exchange might occur through blood vessels even in a sensory‐motor structure such as the colliculus. In a postnatal developmental series of kitten brains we found that enkephalin‐like immunoreactivity was already distinctly compartmental in the intermediate layers at birth and continued to show this distribution throughout postnatal development. By contrast, acetylcholinesterase staining was nearly homogeneous at birth and became compartmental gradually during the first postnatal weeks. Thus, despite the eventual near coincidence of the enkephalinrich and acetylcholinesterase‐rich compartments of the superior colliculus, they mark systems that follow distinct programs of neurochemical development. © Wiley‐Liss, Inc.